In vertebrates, the mechanisms of natural and specific immunity cooperate within a system of host defenses, the immune system, to eliminate foreign invaders. The hypothesis that the immune system ought to be able to recognize tumors and thus could be recruited in the fight against cancer has been a driving force behind outstanding efforts of many immunologists. This approach is attractive because of the unique ability of the immune system to specifically destroy affected cells while mostly sparing normal tissue. Moreover, the initial immune response is known to leave behind a long-term memory that serves to protect from the same disease in the future. No drug treatment for cancer can claim such specificity or memory.
An immunotherapeutic strategy for the treatment of cancer and other diseases or conditions involve one or more components of the immune system to trigger a complex cascade of biological reactions focused on eliminating a foreign molecule from the host. Vertebrates have two broad classes of immune responses: antibody responses, or humoral immunity, and cell-mediated immune responses, or cellular immunity.
Humoral immunity is provided by B lymphocytes, which, after proliferation and differentiation, produce antibodies (proteins also known as immunoglobulins) that circulate in the blood and lymphatic fluid. These antibodies specifically bind to the antigen that induced them. Binding by antibody inactivates the foreign substance, e.g., a virus, by blocking the substance's ability to bind to receptors on a target cell or by attracting complement or the killer cells that attack the virus. The humoral response primarily defends against the extracellular phases of bacterial and viral infections. In humoral immunity, serum alone can transfer the response, and the effectors of the response are protein molecules, typically soluble, called antibodies. Lymphocytes produce these antibodies and thereby determine the specificity of immunity; it is this response that orchestrates the effector limbs of the immune system. Cells and proteins, such as antibodies, that interact with lymphocytes play critical roles in both the presentation of antigen and in the mediation of immunologic functions.
Individual lymphocytes respond to a limited set of structurally related antigens. As noted in more detail below, this function is defined structurally by the presence of receptors on the lymphocyte's surface membrane that are specific for binding sites (determinants or epitopes) on the antigen.
Lymphocytes differ from each other not only in the specificity of their receptors, but also in their functions. One class of lymphocytes, B cells, are precursors of antibody-secreting cells, and function as mediators of the humoral immune response.
Another class of lymphocytes, T cells, express important regulatory functions, and are mediators of the cellular immune response. The second class of immune responses, cellular immunity, involve the production of specialized cells, e.g., T lymphocytes, that react with foreign antigens on the surface of other host cells. The cellular immune response is particularly effective against fungi, parasites, intracellular viral infections, cancer cells and other foreign matter. In fact, the majority of T lymphocytes play a regulatory role in immunity, acting either to enhance or suppress the responses of other white blood cells. These cells, called helper T cells and suppressor T cells, respectively, are collectively referred to as regulatory cells. Other T lymphocytes, called cytotoxic T cells, kill, for example, virus-infected cells or tumor cells. Both cytotoxic T cells and B lymphocytes are involved directly in defense against infection and are collectively referred to as effector cells. There are a number of intercellular signals important to T cell activation. Under normal circumstances an antigen degrades or is cleaved to form antigen fragments or peptides. Presentation of At antigen fragments to T-cells is the principal function of MHC molecules, and the cells that carry out this function are called antigen-presenting cells (APC: including but not limited to dendritic cells, macrophages, and B cells).
The time course of an immune response is subdivided into the cognitive or recognition phase, during which specific lymphocytes recognize the foreign antigen; the activation phase, during which specific lymphocytes respond to the foreign antigen; and the effector phase, during which antigen-activated lymphocytes mediate the processes required to eliminate the antigen-carrying target cells. Lymphocytes are immune cells that are specialized in mediating and directing specific immune responses. T cells and B cells become morphologically distinct only after they have been stimulated by an antigen.
The capture and processing of an antigen by APCs is essential for the induction of a specific immune response. APCs capture antigens via specific receptors, such as Fc receptors or mannose receptors, or the APCs non-specifically phagocytose antigen. The capture through specific receptors is more efficient; antigens can be presented better when in complex with, for example, an antibody. Such a complex can be formed by injecting an antibody to a circulating antigen (e.g., PSA or CA 125), and the immune complexes can be targeted to dendritic cells and macrophages through the Fc-receptors present on these cells. However the high number of Fc receptors on neutrophils may considerably limit this process.
Immunotherapy is based on the principle of inducing or activating the immune system to recognize and eliminate undesirable cells, such as neoplastic cells. The key elements in any immunotherapy is to induce or trigger the host immune system to first recognize a molecule as an unwanted target, and then to induce the system to initiate a response against that molecule. In healthy hosts, the immune system recognizes surface features of a molecule that is not a normal constituent of the host (i.e., is “foreign” to the host). Once the recognition function occurs, the host must then direct a response against that particular foreign molecule.
Both the recognition and the response elements of the immune system involve a highly complex cascade of biological reactions. In most immunologically based disorders, at least one of the steps in the recognition phase, or at least one of the steps in the response phase, are disrupted. Virtually any disruption in either of these complex pathways leads to a reduced response or to the lack of any response. The inability of the immune system to destroy a growing tumor has been attributed, among other factors, to the presence of tumor-associated antigens (TAA) that induce immunological tolerance and/or immunosuppression. For example, in some kinds of cancer, the cancer itself tricks the host into accepting the foreign cancer cell as a normal constituent, thus disrupting the recognition phase of the immune system. The immunological approach to cancer therapy involves modification of the host-tumor relationship so that the immune system is induced or amplifies its response to the TAAs. If successful, inducing or amplifying the immune system can lead to tumor regression, tumor rejection, and occasionally, to tumor cure.